In the past, thin layers, sheets, and films of natural latex rubber have been widely used for a variety of different products, ranging from decorative articles, protective devices, and many different kinds of medical devices. Such thin rubber articles are often made in seamless form by dipping or otherwise coating a shaped, rigid mold of fixed dimensions, into a liquid latex solution and curing the resulting shaped rubber article.
A large proportion of such manufactured articles are often defective and unusable for their intended purposes, due to the formation of small openings, e.g., pin holes, in the rubber film, as well as thin spots, or weakened areas in the thin rubber layer. Such small openings become enlarged during elastic stretching of the article, and sometimes burst; as does the thin film in the thin spots or weakened areas as the rubber article is stretched.
As a result, such manufacturing imperfections result in undesirably large rejection rates of the resulting products, that in some instances exceed more than 10% of the yield. Particularly in the medical product uses, such imperfections render the articles unsuitable, permitting contaminated body fluids to leak through the small openings in the rubber layer and resulting in the possibility of transmitting infection to persons in contact with the rubber articles.
The mold and techniques disclosed herein provide a method whereby a rubber article is rendered "self-healing" by stretch-coating the article on an "expandable mold". However, the prior art does not teach making such a mold, so that the article can be stretched uniformly (proportionally) in three dimensions, as that would be the case for a shaped (hollow) article. Neither does the prior art disclose a method or apparatus for minimizing the occurrence of the above-mentioned defects in the pre-cure manufacturing stage.
In addition, the mold and techniques disclosed herein provide a method whereby an elastic prophylactic device or the mold of the present invention may be coated with a continuous thin layer of a ductile metal while the device is stretched over a mold. The resulting device can be repeatedly expanded and contracted without fracture or breaking of the thin metal layer.
The physical realities which are encountered in developing a mold to be used to create the above described objects are unique to the process and this invention. For instance, when a shaped, hollow elastic article is sufficiently pressurized to be inflated, the article does not inflate uniformly and proportionally to its shape in its relaxed, uninflated state, but it balloons out, tending to assume a spherical shape. This is the case, for example, with a latex medical glove, or any rubber glove, regardless of its thickness. An additional appreciation of the problem is gained by the experiment wherein two identical rubber balloons, inflated to two different diameters are connected with a tube, whereupon air rushes from the balloon of the smaller diameter to the other of the larger diameter. The quantitative relationship among pressure (P), tension (T) (defined as the force over proportional elongation), and the radius (R) of an inflated elastic sphere is given by the formula: ##EQU1## For an elastic cylinder the formula is: ##EQU2##
These formulas are known as "Laplace's law". Consequently, the intuitive approach to making an expandable mold, of an arbitrary shape, out of an elastic envelope having different curvatures at different regions of its surface, and which can be uniformly expanded (inflated), is precluded by Laplace's law. Only when the elastic envelope has a spherical shape (i.e., a single radius of curvature) and a substantially uniform wall thickness, proportional expansion is possible.
A disadvantage of using customary rigid formers (molds) for making shaped elastic devices, such as latex medical gloves and prophylactics, by the conventional dipping technique, is that the rigid molds cannot be used to prevent or correct the occurrence of manufacturing defects, such as pinholes and thin spots. A rigid mold serves merely as a passive support on which the thin latex film forms, and therefore, it cannot modify the latex film, once formed, in any way.
Natural latex rubber is the most widely used material for many different kinds of protective and medical devices. Latex, however, contains certain undesirable, water soluble proteins which become entrapped in the elastomeric matrix of the finished (vulcanized) devices. Such proteins leach out of the devices in contact with human tissues during their intended use. As reported in the Food and Drug Administration Medical Alert letter of Mar. 29, 1991, such leachable proteins can cause adverse reactions and deaths. The current guidelines to manufacturers for deproteinization of finished latex devices call for their immersion in leaching tanks at elevated temperatures and post-cure processing wherein the devices are washed off-line with hot water. Also, for surface treatment of the cured latex devices with chlorine or other agents which may denature surface constituents such as water soluble proteins to render them harmless. These deproteinization treatments are performed while the latex devices are off their molds and in their relaxed (unstretched) state which cannot facilitate in any way a more efficient removal of solubles from the finished device.
Currently, "dipped" latex devices, such as medical gloves and condoms, are tested for the integrity of their continuous thin latex membrane to provide an opening-free continuous barrier to pathogens, by two methods:
(a) on-line, wherein the ionic current through the devices is monitored while the devices are still on their rigid, electrically conductive, molds; PA1 (b) off-line, wherein a small percentage of the finished devices is selected, by statistical sampling techniques, and the devices are filled with a specified amount of water and are checked for leaks. In their stretched state they may also be tested electrically. PA1 (a) varying the wall-thickness profile of the envelope; and/or, PA1 (b) selecting the limits of the linear elastic range of the envelope's material; and/or, PA1 (c) providing under the envelope an inflatable, non-extensible liner; and/or, PA1 (d) filling the mold cavity with on open-cell foam rubber body in adhesive contact with the inner surface of the envelope, in the expanded state of the mold. PA1 (a) rendering shaped rubber devices "self-healing"; PA1 (b) minimizing pinholes and thin spots in rubber latex dipped devices during the pre-cure stage; PA1 (c) removal of solubles from finished, latex devices; and PA1 (d) on-line electrical testing of finished, latex devices in their stretched state.
The prior art does not provide a method by which on-line testing of the finished devices is performed with simultaneous three-dimensional stretching of the devices. These are the conditions under which pinholes and other manufacturing defects are enlarged and become evident in seemingly intact devices. Such defects may not be detected in either the off-line or on-line tests such as electrical tests when the products are in their unstretched state.
An additional disadvantage of rigid molds conventionally used in the manufacture of dipped latex devices is that they are subject to damage with dents and scratches, producing defective devices.